The present invention relates to a stretched, rough electrical insulating film of polypropylene, comprising zones having different degrees of roughness which lie side by side and form fine channels between each other. The film is particularly suitable for the fabrication of impregnated capacitors and for the sheathing of cables. The present invention relates also to a process for the manufacture of such film.
The materials presently used in the construction of impregnated capacitors are normally combinations of paper-aluminum, paper-polypropylene film-aluminum, or paper-metallized polypropylene film. The capacitors prepared from such material combinations are, however, relatively voluminous. In view of the constantly decreasing dimensions of electrical components, development tends toward capacitors which are constructed of polypropylene films and aluminum or of metallized polypropylene films only and which are called "all-film capacitors".
As compared with the hitherto used insulating paper, polypropylene films have an excellent insulation resistance and outstanding dielectric properties. Conventional polypropylene films, however, are very smooth and tend to cling to each other. In a capacitor wound from such a film, the air entrapped between the film layers cannot be expelled or can be expelled only incompletely by impregnation. A capacitor of this kind is useless because, even at low voltages, short circuits and punctures will occur in the non-impregnated areas.
In order to improve the impregnatability of capacitors comprising polypropylene films, processes have been developed, wherein these films are roughened by systematically influencing the morphology (.beta.-.alpha. conversion of crystallites). Such processes are described in German Offenlegungsschriften Nos. 2,553,693, 2,601,810 and 2,722,087. Although it has been possible to improve the impregnation of capacitors produced from films manufactured according to these processes, non-impregnated areas cannot be completely eliminated and, as a consequence, the above-described disadvantages experienced with smooth films will still occur. This is due to the fact that these films have completely irregular surface structures over their entire surfaces. During impregnation, the flow paths for the impregnating medium develop purely accidentally, so that non-impregnated areas and thus air pockets cannot be excluded with certainty.